They didn't need Baker. If someone else saw Oswald in the lunch room shortly after the shooting, it would have been just as good. Of course, it couldn't be TOO soon after the shooting because obviously it had to look like he had sufficient time to get down there from the 6th floor.
So, there was some delicacy involved, but the witness did not have to be a cop. Most of the time, witnesses aren't cops. Right? And when you consider that Oswald admitted to Fritz seeing an officer in the lunch room, then why wouldn't he admit seeing someone else there- if he did? Of course, he would have. He was innocent. He had no reason to lie about seeing someone in the lunch room.
So, they didn't need Baker. Not at all.
In all probability, Baker just came along. Nobody tapped him on the shoulder that morning and said, "Hey, you've got an important role to play today, and I don't mean being a motorcycle escort." Nobody said squat to him.
I can't say whether Truly made any effort to steer Baker to Oswald. It doesn't appear so.